It is already known to administer a percutaneous absorption type drug through the skin by, for example, adhering to the skin or mucosa a polymer containing a percutaneous absorption type drug, and many methods thereof have been proposed in various patents, literatures, etc. The typical method comprises forming on a suitable backing an adhesive layer comprising a polymer which comprises a rubber or acrylic resin based pressure-sensitive adhesive admixed with a percutaneous absorption type drug.
The actual concentrations of the percutaneous absorption type drugs present in the adhesives have been equal to or less than the saturated solubility thereof in the pressure-sensitive adhesives. The reason for this is that even if the drug in excess of the saturated solubility is simply incorporated in the adhesive, large-sized crystal particles thereof are formed and remain in the adhesive layer without being absorbed through the skin.
On the other hand, a method has been proposed to absorb a drug through the skin without loss by adding a percutaneous absorption type drug to a pressure-sensitive adhesive in an amount greater than the saturated solubility as disclosed in, for example, U.S. Pat. No. 4,719,226.
According to this method, the amount of drug in excess of the solubility is dispersed in the adhesive layer as recrystallized fine particles, thereby allowing the drug to be absorbed through the skin without loss for a predetermined period of time.
Besides, it is known that the adhesive strength of the pressure-sensitive adhesive to the skin decreases with the lapse of time in addition to external factors. Therefore, if a pressure-sensitive adhesive having a low viscosity is used, the desired adhesion effect to the skin can be obtained, but there is the disadvantage that part of the adhesive remains on the skin (hereinafter referred to as "adhesive remaining problem"). On the other hand, if a pressure-sensitive adhesive having a high viscosity is used, the adhesive does not remain on the skin, but in some cases, the desired adhesion effect to the skin is obtained only for a short period of time.
Under the above circumstance, if the drug in excess amount above the saturated solubility in a pressure-sensitive adhesive is dispersed in the adhesive in the form of recrystallized fine particles, in the event that the pressure-sensitive adhesive has a high visocisity, there are the same disadvantages as described above and the edge peeling of a preparation occurs; and in the event that the adhesive has a low viscosity, since the excess amount of the percutaneous absorption type drug is dispersed in the form of recrystallized fine particles, the drug does not exhibit the function as fillers and the adhesive remaining problem still occurs.